Silicone-containing elastomer and process for the preparation thereof

ABSTRACT

A novel elastomeric composition of matter comprises a suitable elastomer, a masterbatch consisting of a silicone compound in an amount of from about 10 to about 50 parts by weight based upon the weight of the elastomer, titanium dioxide, and clay. A process for the preparation of a silicone-containing elastomeric composition of matter is provided and includes the steps of masterbatching a silicone compound, clay and titanium dioxide to form a dispersion, adding the dispersion to the elastomer and thereafter forming and curing a desired product therefrom. The composition is useful in the manufacture of rubber goods such as surgical and medical products which can be single or multi-layered.

TECHNICAL FIELD

The present invention relates to a novel elastomeric composition ofmatter, which contains a relatively high amount of a silicone compound,and a process for making the composition. The composition isparticularly suitable for the manufacture of medical and surgicalinstruments such as endo-tracheal tubes, intravenous tubing andspecifically, Foley catheters, and it can easily be utilized inconventional multiple-dip manufacturing processes.

Catheters, as well as other medical and surgical instruments, havecustomarily been manufactured from natural latex primarily because theproducts have a low permeability, they are easily fabricated and arerelatively inexpensive. The rubber composition selected must not only besusceptible to sterilization and other cleaning operations, but it mustalso be able to function satisfactorily when in contact with the bodyand its fluids. Whether the use is of a prolonged indwelling nature orshort term and repeated, a catheter must remain impermeable to water andurine. Swelling of the rubber composition due to absorption of fluid, aswell as deposition of various body materials are common disadvantagessought to be avoided.

Frequent attempts to improve the catheters have been made by varying thecomposition. Greater lubricity, for instance, can be achieved byutilization of silicone compounds which can result in less tissueirritation, resistance to the adhesion of calculi and longer servicelife.

Moreover, insertion and removal of catheters containing silicone isfacilitated due to the smooth, oily-type surface thereof. It has beenfound that while pure silicone rubber possesses these desirablefeatures, it often lacks sufficient strength to produce usefulinstruments. As a further disadvantage, the manufacture of such productsvia multiple dipping is not possible, requiring separate cementing ofmolded catheter tips and funnels to the drainage and inflation tubes.Incorporation of the silicone compound in the natural rubber latex blendimparts the advantages of the silicone to the latex and yet avoids theweaknesses inherent in the use of an all silicone rubber.

BACKGROUND ART

While it has been known to combine silicone emulsions with naturalrubber latex in minute amounts, on the order of 0.025 parts by weight ofthe latex, for purposes of defoaming or reducing adhesioncharacteristics of the latex foam, combinations of larger amounts havenot been successful inasmuch as they are merely mixtures of thecomponents which tend to separate or "bleed out" of the latex withmigration to the surface. Such bleeding presents problems in themanufacture and use of multi-layered products such as de-lamination,local areas of surface tack and in the case of Foley catheters,weaknesses in the inflatable balloon.

Natural rubber latex compounds suitably employed for the manufacture ofhigh quality surgical tubing and catheters comprise 96% rubber by weightand 4% of other ingredients such as curatives, fillers, anti-oxidantsand stabilizers. These formulations have not heretofore accepted evensmall amounts of silicone, i.e., 3 to 5% by weight per hundred parts ofrubber, and produced finished products having satisfactory functionalcharacteristics.

One U.S. patent, No. 3,962,519, does provide for incorporating siliconecompounds in natural and synthetic rubber and relies upon the bleedingor migration of the silicone compound to the external surface of theinstrument to provide resistance to unwanted adhesion and waterrepellency. The patent discloses employment of silicone compounds havinga molecular weight of more than 90,000 and states that the amountpresent can range from about 0.1 to 10% by weight based upon the weightof the rubber. Of course, due to the migratory behavior of the silicone,incorporated according to the disclosure of this patent, relatively highamounts of silicone cannot be employed without causing separation of thesilicone from the rubber latex and concommitant failure of the product.Notwithstanding the disclosure and claims of this prior patent, I havenot found it possible to incorporate more than about 1% of a siliconecompound of the type disclosed therein and form a satisfactory product.

DISCLOSURE OF THE INVENTION

It is therefore an object of the present invention to provide anelastomeric composition comprising from about 10 to about 50 percent byweight of a silicone compound, based upon the weight of the elastomerwhich composition is essentially free from deleterious phasing.

It is another object of the present invention to provide medical andsurgical instruments from the novel elastomer disclosed herein whichhave greater water repellency, smoother surfaces, higher lubricity andlonger life than existing instruments.

It is yet another object of the present invention to provide anelastomeric composition of natural rubber latex and a silicone compoundthe combination of which eliminates known disadvantages of the separatecomponents without loss of the advantages thereof.

It is a further object of the present invention to provide a process forthe preparation of an elastomeric composition comprising from about 10to about 50 percent by weight of a silicone compound, based upon theweight of the elastomer.

It is still another object of the present invention to provide a processfor the preparation of a silicone-containing elastomeric compositionthat is essentially free from deleterious phasing, allowing themanufacture of useful, long-lasting surgical and medical instrumentstherefrom via conventional processes.

It is yet another object of the present invention to provide surgicaland medical instruments, such as Foley catheters, from thesilicone-containing elastomeric compositions disclosed herein havingfrom about 10 to about 50 percent of a silicone compound, based upon theweight of the elastomer.

These and other objects of the present invention, together with theadvantages thereof over the prior art, which shall become apparent fromthe specification that follows, are accomplished by the invention ashereinafter described and claimed.

In general, the preferred elastomeric composition of the presentinvention comprises a rubber latex and from about 20 to about 85 partsby weight of a masterbatch which includes, based upon the weight of theelastomer, from about 10 to about 50 parts of a silicone compound, fromabout 5 to 15 parts by weight of titanium dioxide, and from about 5 to20 parts by weight of clay. The silicone-containing composition isprepared by the process of masterbatching a silicone compound, clay andtitanium dioxide to form a dispersion, adding the dispersion to theelastomeric compound with stirring and thereafter forming and curing adesired product therefrom. Surgical and medical products such as Foleycatheters can be formed from the silicone-containing elastomericcomposition of the present invention via conventional processingtechniques for natural and synthetic rubber.

PREFERRED MODE FOR CARRYING OUT THE INVENTION

Formation of the silicone-containing elastomeric composition set forthherein generally includes the use of natural rubber latex and a siliconeemulsion. Although the examples provided herein disclose the use of anatural rubber latex having 56.5% total solids content by weight, it isto be understood that other latexes as well as synthetics and mixturesof the two which have conventionally been employed in the manufacture ofrubber surgical and medical instruments can also be employed.

With respect to the silicone compound, one can select polysiloxaneshaving a viscosity of about 100 centistokes and a viscosity-averagemolecular weight of about 7,000, with dimethylpolysiloxane beingexemplary. One particularly satisfactory silicone compound is producedby the Silicone Products Department of General Electric, and isidentified by the code SM-2064. This compound is an anionic emulsionhaving approximately 50% total solids by weight. It requires atemperature in excess of 260° C. to increase its molecular weight andtherefore, in curing operations of the type conventionally employed inthe manufacture of rubber surgical and medical instruments, it retainsits low molecular weight of about 7,000.

The amount of the silicone compound added can range from at least 10parts to about 50 parts by weight per hundred parts of rubber (phr).Although the silicone content is significantly higher than has beendemonstrated to be practical heretofore, according to the processdescribed herein, it is homogeneously dispersed throughout the latexcomposition and therefore has a greater tendency than previouscompositions to remain so both during and subsequent to vulcanization.Thus, bleeding or migration of the silicone, commonly referred to asphasing and characteristic of the prior art latex-silicone mixtures, hasbeen reduced by the present invention, allowing much higher amounts ofsilicone incorporation in the rubber latex without eventual separation.It is to be understood that while some controlled migration can beadvantageous, the rate of migration as well as the amount must not be sogreat that separation or settling of the silicone from the elastomerwill occur.

In addition to the conventionally employed antioxidants, pigments,curing ingredients and stabilizers which are incorporated with rubberlatex compositions, practice of the present invention requires thepresence of clay and titanium dioxide. The clay is added as an aqueousdispersion having about 60% total solids by weight. Kaolin is quitesuitable for this purpose and can be added in amounts of from about 5 toabout 20 parts by weight phr with 15 parts being preferred. Titaniumdioxide is also added as an aqueous dispersion, having about 50% totalsolids content, in an amount of from about 5 to about 15 parts phr with5 being preferred.

Not only must the clay and titanium dioxide be present in thecomposition, in order to add the relatively high amounts of siliconedisclosed herein and form a homogeneous, essentially non-separatingproduct, it is necessary to masterbatch the clay, titanium dioxide andsilicone emulsion together prior to their addition to the rubber latex.If added separately, migration and eventual separation of the siliconefrom the latex will occur. In practice, the titanium dioxide and kaolindispersions are combined with high speed stirring followed by theaddition of the silicone emulsion at lower speeds for approximately 10minutes. The resulting masterbatch comprises from about 20 to about 85parts by weight of the composition, based upon the weight of theelastomer, and is added to the rubber latex base formulation utilizingconventional procedures at slow stirring speeds.

In the four examples which follow, 15, 20, 35 and 50 parts phr ofsilicone respectively, were masterbatched with clay and titaniumdioxide, as presented in Table I, and then added to a latex rubberformulation which is given in Table II. Products were subsequently madeby a conventional dipping process, vulcanized and tested with thetesting results appearing in Table III. All parts are expressed in partsby weight phr and may, therefore, alternatively be considered as percentby weight, based upon the weight of the elastomer.

Although physical properties for pure gum catheters are greater thanthose reported hereinbelow, corresponding physical properties for an allsilicone rubber are generally less. While the pure gum possessessignificantly greater strength than necessary, the all silicone rubberis on the weak side for medical and surgical products. As will bereadily determined from the data in Table III, the silicone-containingelastomeric compositions of the present invention have acceptableproperties for medical and surgical products which are conventionallymanufactured from rubber latex. Finished articles produced from thiscompound, multi-layered or single dip, have been found to have largeamounts of silicone distributed homogeneously throughout. No phasing orseparation of the silicone from the rubber latex compound, either duringstorage or processing, has been observed.

                  TABLE I                                                         ______________________________________                                        Example Nos.                                                                           1         2         3       4                                        ______________________________________                                        TiO.sub.2                                                                              5.0       5.0       5.0     5.0                                      Percent Total                                                                 Solids   50.0%     50.0%     50.0%   50.0%                                    Dry Wt.  99.91 gms 99.91 gms 99.91 gms                                                                             99.91 gms                                Wet Wt.  199.8 gms 199.8 gms 199.8 gms                                                                             199.8 gms                                Clay     10.0      15.0      20.0    25.0                                     Percent Total                                                                 Solids   60.0%     60.0% 60.0%                                                                             60.0%                                            Dry Wt.  199.81 gms                                                                              299.72 gms                                                                              399.63 gms                                                                            499.53 gms                               Wet Wt.  333.0 gms 499.5 gms 666.0 gms                                                                             832.5 gms                                G.E. Silicone                                                                 SM-2064  15.0      20.0      35.0    50.0                                     Percent Total                                                                 Solids   50.0%     50.0%     50.0%   50.0%                                    Dry Wt.  299.72 gms                                                                              399.63 gms                                                                              699.35 gms 999.08 gms                            Wet Wt.  599.48 gms                                                                              799.3 gms 1398.78 gms                                                                           1998.25 gms                              ______________________________________                                    

                  TABLE II                                                        ______________________________________                                                        Percent                                                                       Total                                                         Component                                                                             Parts   Solids    C.F. Dry Wt. Wet Wt.                                ______________________________________                                        Rubber  100.0   56.5%     1.027                                                                              1998.13 gms                                                                           3632.0 gms                             latex.sup.(a)                                                                 Zetax.sup.(b)                                                                         0.2     25.0%          5.99 gms                                                                              24.0 gms                               Methyl                                                                        tuads.sup.(c)                                                                         0.3     35.0%          5.99 gms                                                                              17.1 gms                               ZnO     0.4     50.0%          7.99 gms                                                                              16.0 gms                               ______________________________________                                         .sup.(a) Natural rubber latex, contains 0.7 parts sulphur.                    .sup.(b) Registered trademark of Goodyear Tire and Rubber Company for zin     2mercaptobenzothiazole.                                                       .sup.(c) Registered trademark of R.T. Vanderbilt Co., Inc. for                tetramethylthiuram disulfide.                                            

                                      TABLE III                                   __________________________________________________________________________    Example        600% Modulus                                                                           Tensile Strength                                      No.  Condition                                                                           Gauge                                                                             (psi)                                                                             (Kg/cm.sup.2)                                                                      (psi)                                                                             (Kg/cm.sup.2)                                                                      Elongation                                   __________________________________________________________________________    1    Unaged                                                                              0.0181/2                                                                          920 64.68                                                                              2780.sup.(b)                                                                      195.45.sup.(b)                                                                     815.sup.(b)                                       Aged.sup.(a)                                                                        0.0191/2                                                                          1080                                                                              75.93                                                                              2790                                                                              196.16                                                                             770                                          2    Unaged                                                                              0.0183/4                                                                          910 63.98                                                                              2020                                                                              142.02                                                                             775.sup.(b)                                       Aged.sup.(a)                                                                        0.0173/4                                                                          1040                                                                              73.12                                                                              2280                                                                              160.30                                                                             730                                          3    Unaged                                                                              0.0201/2                                                                          780 54.84                                                                              1390                                                                              97.73                                                                              730                                               Aged.sup.(a)                                                                        0.0181/2                                                                          890 62.57                                                                              1410                                                                              99.13                                                                              690                                          4    Unaged                                                                              0.0181/2                                                                          610 42.89                                                                               990                                                                              69.60                                                                              700                                               Aged.sup.(a)                                                                        0.020                                                                             310 21.8  690                                                                              48.51                                                                              660.sup.(b)                                  __________________________________________________________________________     .sup.(a) Specimens air oven aged for 168 hours at 70° C.               .sup.(b) Average value                                                   

The compositions disclosed herein are particularly suitable for themanufacture of Foley catheters made by a standard multiple dip processto build wall thickness around the drainage lumen and to form theinflatable balloon. Because the silicone compound can be considered tobe essentially non-separable from the rubber latex, adhesion betweensuccessive layers is not hindered as is experienced when large amountsof silicone migrate to the exterior of a layer. Foley catheters madefrom the elastomeric composition disclosed herein have been tested inpatients for indwelling periods of time of up to seven weeks withoutirritation to the patient or physical malfunction or deterioration ofthe catheter. In other tests, the inflation balloons of catheters havebeen filled with water and allowed to rest on a table for seven weeks.During this time no leakage of water was observed. Significance of thewater inflation test is in the fact that all silicone catheters arepermeable to water.

Thus, it can be seen that by employing the process disclosed herein, itis possible to prepare a novel elastomer having from about 10 to about50 parts phr of a silicone compound which remains homogeneouslydistributed throughout the rubber latex even after vulcanization. Aswill be apparent to those skilled in the art, the composition of thenovel elastomer disclosed herein can be selected according toavailability of ingredients, desirability of process and nature of theend product. The methods generally available for making known surgicaland medical instruments can be practiced with the elastomer disclosedherein which in turn can enable the worker to achieve the objects of theinvention.

It is, therefore, believed that the preparation and use ofsilicone-containing elastomers disclosed herein can be determinedwithout departing from the spirit of the invention herein disclosed anddescribed, the scope of the invention being limited solely by the scopeof the attached claims.

I claim:
 1. A novel homogenous silicone-containing stable elastomericcomposition of matter comprising:a natural rubber latex; and from about20 to about 85 parts by weight, based upon the weight of the rubber insaid latex, of a masterbatch consisting of a dimethylpolysiloxanecompound having a viscosity average molecular weight of about 7,000 anda viscosity of about 100 centistokes, said compound being present in anamount of more than 10 to about 50 parts by weight, based upon theweight of the rubber in said latex, and being substantially free fromphasing; from about 5 to 15 parts by weight of titanium dioxide, basedupon the weight of the rubber in said latex; and, from about 5 to 20parts by weight of clay, based upon the weight of the rubber in saidlatex.
 2. A novel elastomeric composition of matter, as set forth inclaim 1, containing 50 parts by weight of said dimethylpolysiloxanecomposition and having a tensile strength of at least 690 psi (48.5Kg/cm²) at 660% elongation and a 600% modulus of at least 310 psi (21.8Kg/cm²) after aging for one week at 70° C.